Ch 9, Lesson D, Page 1 - Introduction on Non-Ideal Vapor Power Cycles

Every cycle that we have discussed up to this point in this chapter has been ideal.

In this case, ideal means that the pump and compressor are isentropic and that the boiler, condenser and all pipes in the process are internally reversible.

In this lesson, we will determine how the performance of a cycle made up of real, irreversible processes differs from the ideal Rankine Vapor Power Cycle.

First, we will qualitatively assess the impact of non-ideal behavior by considering the paths of real cycles on TS Diagrams.

Then we will turn quantitative and perform a thermodynamic analysis of a real that includes all of the internal irreversibilities that we discuss.

The thermodynamic analysis will show us that all of the many and varied irreversibilities associated with real processes ultimately contribute to the total entropy generation and lost work for the cycle.

If you understand where this lesson is going, then let’s get started.

The 1st question that comes to mind when pondering real cycles is, “Why are they less efficient than the Rankine Cycle ?”

That seems like a good place to start.

There are 4 main reasons that real vapor power cycles are less efficient or more irreversible than the ideal Rankine Cycle.

The first reason is that heat is lost from all devices and pipes that are at temperatures greater than the temperature of the surroundings. There are no adiabatic processes and all heat transfer is irreversible.

The next two reasons that all real cycles are irreversible relate to friction.

When different regions of a fluid are moving at different velocities friction occurs as adjacent layers slide past each other.

Friction also occurs at the interface between fluids and solid surfaces that are moving relative to each other.

This occurs in the pump and turbine, as well as in all of the pipes connecting the 4 processes that make up the cycle.

Friction converts the kinetic energy associated with a moving fluid or a moving solid, as in the pump or turbine, into internal energy. This conversion is irreversible. We cannot convert heat completely back into kinetic energy any more than we can completely convert heat into work.

The last reason that a real vapor power cycle is less efficient than an ideal one stems from a practical aspect of pump operation.

In lesson B, we learned that pumps become less efficient and can be severely damaged if any gas is in the feed stream.

It turns out that if a saturated liquid is fed to a pump, a small amount of the liquid will vaporize and damage the pump.

This phenomenon is called cavitation and you will learn more about it in you fluid mechanics course.

All we need to know now is that, for practical reasons, the pump feed must be a subcooled liquid.

This means that QC will be larger and therefore the efficiency will be lower.

All of these phenomena lead to the same conclusion: the efficiency of a real vapor power cycle will be less than the efficiency of an ideal Rankine Vapor Power Cycle.

Now, let’s see how these phenomena effect the path for the cycle on the TS Diagram.